RF amplifiers are utilized for a variety of applications in wireless communication systems, such as to amplify or transmit a downlink signal at a base station. As a result, the amplifiers must be able to handle the power requirements associated with such wireless communication systems. Particularly, amplifiers used for applications having high peak-to-average signal ratios must be able to handle or deliver significant peak power levels above their normal or average load. For example, in order to amplify a signal with a 10 dB peak-to-average ratio, while creating a minimal amount of nonlinear distortion, an amplifier must be capable of delivering around 200 watts of power to generate an output signal with an average power of 20 watts.
Generally, the transistors used for the amplification in RF amplifiers actually run most efficiently when they are outputting close to their maximum power capability, or are operating in a saturated mode. However, the closer to saturation an amplifier is operated, the greater the amount of nonlinear distortion it outputs. Therefore, it has become standard practice to decrease or “back off” the amplifier power output until the nonlinear distortion is at an acceptable level. As a result, for handling high peak-to-average signal ratio applications in an amplifier, several amplification devices are usually operated together, and the multiple amplification devices have their outputs combined. In such an amplifier, the devices or sub-amplifiers are operated inefficiently most of the time at low power in order to be able to deliver, somewhat more efficiently, the maximum power for the relatively short period of time when the high peaks in the signal are to be amplified.
Apparatuses and techniques have been developed to improve linearity for both low level and high level input signal operation in order to more efficiently deliver the desired power requirements for certain wireless communication applications. One particular apparatus is the Doherty amplifier, which improves amplifier efficiency by effectively reducing the amplifier's saturated power level when the signal level is low, yet quickly ramping up to full power capability when the signal peaks demand it. A basic Doherty amplifier design is shown in FIG. 1, and consists of an input splitter 24, carrier amplifier 20, peaking amplifier 22 and output combiner/impedance transforming network 26.
At low input signal levels the peaking amplifier is essentially turned off, not contributing to the output. The Doherty amplifier dynamically adjusts to handle the low input signal level. More specifically, with low input signal levels, the action of the output combiner/impedance transforming network 26 causes a load to be present at the carrier amplifier 20 output that causes it to operate more efficiently at the low signal levels. However, the modified load at the carrier amplifier output results in an increase in signal delay through the carrier amplifier 20 from normal operation.
As the input signal level increases, the peaking amplifier 22 contributes more to the overall output signal. At the same time, the output combiner/impedance transforming network 26 causes the carrier amplifier load to gradually change to a level that allows the carrier amplifier to output maximum power. The delay through the carrier amplifier is also affected. More specifically, at full power, the carrier amplifier delay has been reduced to its normal level. Thus the delay of the overall Doherty amplifier has decreased with an increasing input signal.
The varying delay through the carrier amplifier 20, however, causes some undesirable characteristics in the operation of the Doherty amplifier design. Specifically, the variation in the delay of the Doherty amplifier resulting from the variation in the input signal level results in changes in the amplifier's AM-to-PM (AM/PM) characteristic as the frequency of the operation is changed. The AM/PM characteristic of an amplifier is the change in the amplifier's phase shift as the input power to the amplifier is changed. While amplifiers utilizing common Class-AB operation have relatively small changes in the AM/PM over a given frequency range, Doherty amplifiers have a significantly larger change in the AM/PM. A typical Doherty amplifier has the characteristic as generally depicted in FIG. 2.
The AM/PM variation causes distortion issues. Specifically, these changes in the AM/PM characteristic cause the inter-modulation (IM) distortion of an optimized Doherty amplifier to degrade as the frequency of operation increasingly deviates from the center frequency of the operational band. This may cause an amplifier to fail a specification at the band edge, or result in a decreased margin to a specification for a Doherty amplifier.
Such a variation in the IM distortion causes other problems as well. In some applications, it is desirable to improve an amplifier's linearity through the use of pre-distortion. Implementation of the pre-distorter in an amplifier is made more complicated by the presence of changing AM/PM distortion, which is a form of memory. While pre-distorters can be implemented to correct for this form of memory, minimizing the amount of memory usually results in a more effective system, and therefore, is a desired solution.
Accordingly, it is desirable to improve the amplification schemes for RF applications associated with high peak-to-average signals ratio. It is also further desirable to address the drawbacks in the prior art by providing efficient and linear amplification with minimal distortion, during both low power and high power peak requirements. It is further desirable to minimize the effects of frequency variations on RF amplification schemes for high peak-to-average signal ratios. These, and other objectives, are addressed by the invention described and claimed herein.